The present invention relates to an optical writing and reading system having a laser diode and a multibeam optical head. The optical writing and reading system optically writes, reads, and deletes information by irradiating convergent rays from a light source onto an optical writing medium.
Along with the development of information processing systems in recent years, it has been strongly desired that a high transfer data rate be attained in optical writing and reading systems, in addition to realization of a large capacity, whereby the writing and reading of information is accomplished. Various efforts have been made towards this goal (as observed in 4p-ZD-1, 4p-ZD-2, etc., read at Extended Abstracts, The 49th Autumn Meeting, 1989, The Japan Society of Applied Physics). In this regard, the adoption of a multibeam optical head has been proposed in which a plurality of writing and reading converging light beam spots are used in the optical head, to obtain the high transfer data rate.
An example of this type of multibeam optical head is disclosed, for example, in Japanese Patent Laid Open No. 117744/ 1986.
As shown in FIG. 1, such an optical head comprises a laser diode array 100 having a plurality of laser diodes therein, each diode independently controllable for its light emission. The diodes in the laser diode array 100 are arranged linearly in a straight line and spaced from one another at an equal interval, to form a unified structure. The condenser lens 101 converts the emitted light rays from the laser diode array 100 into collimated parallel rays. An object lens 102 converges the collimated parallel rays into a plurality of beam spots. A polarizing beam splitter 103 separates incident rays from reflected rays. A plurality of beam splitting and detecting means 104 detects signals by splitting the reflected rays from the plurality of beam spots. A photo-detector 105 detects the position of one track. A pair of focusing error detecting systems 106 detect focusing errors.
Referring to FIG. 2, with this construction, it is possible to perform the writing or reading of information on a plurality of tracks 111 at the same time, by irradiating a plurality of beam spots 107, 108, and 109 (three spots in the example shown in FIG. 2) onto the plurality of tracks 111 on an optical disk 110 (shown in FIG. 1) at the same time.
Referring to FIG. 3, in the above multibeam optical head, if an attempt is made to arrange the plurality of beam spots 107, 108, and 109 linearly spaced from one another at an equal interval along a line in a radial direction A of the optical disk 110, it would be necessary to make the spacing between adjacent ones of the plurality of laser diodes arranged in the laser diode array very small due to such consideration as magnification of the optical system.
As a result, it would virtually be impossible to manufacture such a laser diode array. Therefore, as shown in FIG. 4, a plurality of beam spots 112 through 120 are arranged in a straight line, and the straight line is inclined at a prescribed angle .theta. with respect to the radial direction A of the optical disk 110, such that the space intervals between the adjacent laser diodes in the laser diode array 100 be set at a value practical for manufacture of the laser diode array.
However, as mentioned above, in the above multibeam optical head, the plurality of beam spots 112 through 120 are linearly arranged on a straight line and spaced at an equal interval, and the straight line is inclined at the angle .theta. with respect to the radial direction A of the optical disk 110, as shown in FIG. 4. As a result, it would not be possible to position all of the plurality of beam spots 112 through 120 on the tracks 111 of the optical disk 110, since each of the tracks 111 comprises an arc having a prescribed radius of curvature, resulting that the beam spots on both ends of the straight line are positioned off the tracks 111.
Consequently, it would not be possible that all the beam spots track their respective tracks 111 at the same time with accuracy even if a tracking servo is applied to the optical head. This creates a serious problem in that the writing or reading of information could not be performed on the plurality of tracks 111 at the same time.